Methionine adenosyltransferase (MAT) also known as S-adenosylmethionine synthetase is a cellular enzyme that catalyzes the synthesis of S-adenosyl methionine (SAM or AdoMet) from methionine and ATP and is considered the rate-limiting step of the methionine cycle. SAM is the propylamino donor in polyamine biosynthesis and the principal methyl donor for DNA methylation and is involved in gene transcription and cellular proliferation as well as the production of secondary metabolites.
Two genes, MAT1A and MAT2A, encode two distinct catalytic MAT isoforms. A third gene, MAT2B, encodes a MAT2A regulatory subunit. MAT1A is specifically expressed in the adult liver, whereas MAT2A is widely distributed. Because MAT isoforms differ in catalytic kinetics and regulatory properties, MAT1A-expressing cells have considerably higher SAM levels than do MAT2A-expressing cells. It has been found that hypomethylation of the MAT2A promoter and histone acetylation causes upregulation of MAT2A expression.
In hepatocellular carcinoma (HCC), the downregulation of MAT1A and the up-regulation of MAT2A occur, which is known as the MAT1A:MAT2A switch. The switch, accompanied with up-regulation of MAT2B, results in lower SAM contents, which provide a growth advantage to hepatoma cells. Because MAT2A plays a crucial role in facilitating the growth of hepatoma cells, it is a target for antineoplastic therapy. Recent studies have shown that silencing by using small interfering RNA substantially suppresses growth and induces apoptosis in hepatoma cells. See, e.g., T. Li et al., J. Cancer 7(10) (2016) 1317-1327.
It has been reported by Marjon et al (Cell Reports 15(3) (2016) 574-587) that cancer cell lines that are MTAP deficient are particularly sensitive to inhibition of MAT2A. MTAP (methylthioadenosine phosphorylase) is an enzyme widely expressed in normal tissues that catalyzes the conversion of methylthioadenosine (MTA) into adenine and 5-methylthioribose-1-phosphate. The adenine is salvaged to generate adenosine monophosphate, and the 5-methylthioribose-1-phosphate is converted to methionine and formate. Because of this salvage pathway, MTA can serve as an alternative purine source when de novo purine synthesis is blocked, e.g., with antimetabolites, such as L-alanosine.
Many human and murine malignant cells lack MTAP activity. MTAP deficiency is not only found in tissue culture cells but the deficiency is also present in primary leukemias, gliomas, melanomas, pancreatic cancers, non-small cell lung cancers (NSLC), bladder cancers, astrocytomas, osteosarcomas, head and neck cancers, myxoid chondrosarcomas, ovarian cancers, endometrial cancers, breast cancers, soft tissue sarcomas, non-Hodgkin lymphomas, and mesotheliomas. The gene encoding for human MTAP maps to region 9p21 on human chromosome 9p. This region also contains the tumor suppressor genes p16INK4A (also known as CDKN2A), and pl5INK4B. These genes code for p16 and p15, which are inhibitors of the cyclin D-dependent kinases cdk4 and cdk6, respectively.
The pl6INK4A transcript can alternatively be ARF spliced into a transcript encoding pl4ARF. pl4ARF binds to MDM2 and prevents degradation of p53 (Pomerantz et al. (1998) Cell 92:713-723). The 9p21 chromosomal region is of interest because it is frequently homozygously deleted in a variety of cancers, including leukemias, NSLC, pancreatic cancers, gliomas, melanomas, and mesothelioma. The deletions often inactivate more than one gene. For example, Cairns et al. ((1995) Nat. Gen. 11:210-212) reported that after studying more than 500 primary tumors, almost all the deletions identified in such tumors involved a 170 kb region containing MTAP, pl4ARF and Pl6INK4A. Carson et al (WO 99/67634) reported that a correlation exists between the stage of tumor development and loss of homozygosity of the gene encoding MTAP and the gene encoding p16. For example, deletion of the MTAP gene, but not pl6INK4A was reported to be indicative of a cancer at an early stage of development, whereas deletion of the genes encoding for p16 and MTAP was reported to be indicative of a cancer at a more advanced stage of tumor development. Garcia-Castellano et al reported that in some osteosarcoma patients, the MTAP gene was present at diagnosis but was deleted at a later time point (Garcia-Castellano et al., supra).